Disc drive memory systems have been used in computers for many years for storage of digital information. Such information is recorded on concentric memory tracks of a magnetic disc medium, the information being stored in the form of magnetic transitions of the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of read/write head support on a pivoting arm which moves radially over the surface of the disc. The read/write heads, or transducer, must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of the information. Thus, no tilt or wobble of the disc can be tolerated.
During operation, the discs are rotated at a very high speed within an enclosed housing by means of an electric motor generally located inside the hub or below the discs. The typical motor in current disc drive design is known as an in-hub or in-spindle motor. Such in-spindle motors, in order to minimize the size of the motor, incorporate a hydrodynamic bearing which supports the rotor for rotation around the shaft. Such hydrodynamic bearing are disclosed, for example, in U.S. patent application Ser. No. 08/472,338, filed Jun. 7, 1995, U.S. Pat. No. 5,685,674 entitled "SINGLE PLATE HYDRODYNAMIC BEARING WITH SELF-BALANCING FLUID LEVEL AND FLUID CIRCULATION" by Hans Leuthold, et al. assigned to the Assignee of the present invention and incorporated herein by reference.
In such bearings, a lubricating fluid functions as an actual bearing surface between a stationary base or housing and the rotating spindle or hub and the surrounding stationary portion of the motor. Over time, it is possible that the physical surfaces of the spindle and housing could contact one another, leading to an increased wear and eventual failure of the bearing system. Equally seriously, loss of the seal holding the lubricant within the bearing system or failure to control the lubricant level, or evaporation of the lubricant, or deterioration of the quality of the lubricant, could also cause the effectiveness of the bearing to diminish as the rotating part would no longer be held stable relative to the stationary part. This lack of stability would, over time, translate into what is termed "non-repeatable runout" in the rotating disc, making it difficult for the transducer to follow the tracks on which data is to be read and written. Thus, it is important to develop methods and systems for monitoring the performance of the hydrodynamic bearing.
A further critical issue in the design of hydrodynamic bearings is to maintain the stiffness of the hydrodynamic bearing. The stiffer the bearing, the higher the natural frequencies in the radial and axial direction of the motor, so that the more stable is the track of the disc on which reading and writing occurs. Thus, monitoring of the stiffness of the bearing is very important in evaluating the life of the hydrodynamic bearing so that the rotating load or disc is stably and accurately supported without wobble or tilt.
Yet, another problem with disc drive motors using hydrodynamic bearings is sensitivity to shock because of the lack of mechanical contact between the relatively moving parts. In view of this, a sensitive shock detector must be included in such disc drives, to interrupt data recording until the shock has passed. However, such shock detectors constitute extra expense and are not easily calibrated with the operating status of the hydrodynamic bearing.
A further desirable feature in disc drive motors is monitoring of operating temperature. However, this has also been difficult to achieve without adding extra components to the motor or disc drive.